"No matter how much you had your windows down or the air conditioner on or any other driving conditions, you could always measure tobacco smoke and in most cases you could measure very high concentrations," Dr. Ana Navas-Acien of Johns Hopkins Bloomberg School of Public Health in Baltimore, one of the researchers on the study, told Reuters Health.

While scientists have tested secondhand smoke exposure in automobiles before, Navas-Acien noted in an interview, they had only done so in the lab. "We wanted to evaluate what those levels could be under ... normal, real-world driving conditions," she said.

To do so, she and her colleagues recruited 17 smokers and five non-smokers, all of whom drove their own car to and from work every day. The researchers planted nicotine samplers in the front passenger seat headrest and in the back seat of each car for 24 hours. They measured nicotine because it is a highly accurate, easy-to-measure gauge of second-hand smoke levels, Navas-Acien said, although cigarette smoke contains many other harmful substances.

Nicotine levels were undetectable in the non-smokers' cars. But for smokers' cars, concentrations averaged 9.6 micrograms of nicotine per cubic meter, much higher than concentrations typically measured in places-public or private-where smoking is allowed. And for every cigarette a person smoked, the air nicotine concentration doubled.

"This is because the car is a very small place," Navas-Acien noted. The results, she and her colleagues conclude in the journal Tobacco Control, "support the need for education measures and legislation that regulate smoking in motor vehicles when passengers, especially children, are present."

Risks for kids can include worsening of asthma, ear infections, and more, the researcher said. In addition to legislation, she noted, education is important, because many parents who smoke may not be aware of the risk of second-hand smoke exposure to their young passengers.

Plus, other research has shown that smokers have a higher rate of accidents, and smokers get less for their cars when they sell them, the authors note.